1. Field of the Invention
This invention relates to chemical vapor deposition (CVD) of thin metal films, and in particular to the catalyzed removal of heteroatoms during the deposition of metals from precursor film forming metal compounds in the presence of hydrogen.
2. Description of the Prior Art
The deposit of thin coatings of metals onto various substrates are important in several industries. For example, one of the most important applications is in the production of integrated circuits in the microelectronics industry. In this respect, one of the most important criteria is the purity of the deposited thin metal film. The presence of even small amounts of carbon, oxygen and other heteroatom contaminants can markedly affect the performance of the finished electronic component.
The term "heteroatom" as used herein and in the appended claims is meant to include all atoms except metal atoms.
Various methods have been used for purposes of depositing a thin metal coating onto a substrate including for example precipitation from liquid solution, sputtering, and chemical vapor deposition (CVD), and plasma enhanced CVD.
In addition to purity, it is often desirable to selectively deposit a metal film on a portion of the surface of a semiconductor or other electronic component. This is particularly the case for purposes of providing interconnects to various circuit elements which require selective deposition in small voids in the substrate surface of the electronic component.
The use of liquid solution precipitation of precursor compounds followed by metal deposition has some advantages. However, it has generally been unsatisfactory for purposes of complete and contiguous deposition since it is difficult to insure that the solution will penetrate the small voids which are necessary to insure film quality and proper adherence to the substrate. Additionally, the purity of the deposited coating is often not as high as is necessary for some commercial applications. Sputtering has also been rejected in many instances since the quality of the coating as well as the uniformity of the coating has often been less than commercially acceptable. Also, this method requires that the extra metal introduced be chemically etched away in subsequent treatments.
Chemical vapor deposition, has provided more uniform deposition of thin metal films and is more reliable for conformal coverage in the deposition of metal films on convoluted surfaces. Despite the advantages of CVD, the degree of purity of the deposited metal films has often been less than desired.
Chemical vapor deposition has produced films which have been contaminated with unacceptable levels of carbon and oxygen and other heteroatoms that are derived from decomposition of hydrocarbon and oxocarbon moieties of the volatile CVD precursor compounds.
These precursor compounds are typically organometallic compounds with hydrocarbon, carbonyl, and hydride ligands that are used in organometallic chemical vapor deposition processes (OMCVD).
For instance, the use of metal carbonyl compounds in CVD applications typically introduces carbon and oxygen contaminants. Localized laser heating at high temperatures of small deposited metal "dots" as shown by Houle et al in their work with deposited tungsten appears to essentially clean in the center with impurities on the periphery of the dot area.
The three major chemical processes for achieving CVD can be generally classified as reduction, thermal decomposition, and displacement.
Reduction involves exposing the volatile metal compound during or after deposition on the substrate to hydrogen or equivalent reducing gas. Theoretically, the hydrogen reacts with the nonmetal portion of the compound to yield volatile hydrocarbon byproducts and to leave the metal film behind on the substrate.
Thermal decomposition involves heating the substrate to cause the hydrocarbon portion of the volatile metal compound to decompose to volatile hydrocarbons and leave the surface of the substrate while leaving the metal on the surface.
Displacement involves use of a surface material or surface absorbed species on the substrate to react with the volatile metal compound to yield volatile byproducts and deposit metal on the exposed surface.
All of the above techniques suffer from the introduction of impurities during the decomposition reaction of the precursor metal compound or to produce corrosive byproducts in depositing the metal film.
In recent years, improved purity of thin metal films deposited by CVD involved the use of organometallics as the volatile compound. See Gozum, J. et al., "`Tailored` Organometallics as Precursors for the Chemical Vapor Deposition of High-Purity Palladium and Platinum Thin Films," J. Am. Chem. Soc., 110, 2688 (1988).
In the above referenced article bis(allyl)palladium, bis(2-methylallyl)palladium, and (cyclopentadienyl)(allyl) palladium were investigated for CVD at 250.degree. C. and 10.sup.-4 Torr. The cyclopentadienyl compound yielded CVD palladium films having about 5 mol % residual carbon as a contaminant. However, it was also found that at a similar temperature and pressure, the cyclopentadienyl complex CPPtMe.sub.3 [(cyclopentadienyl)(trimethyl)platinum(IV)] produced high quality platinum films that were not significantly contaminated with carbon. In some cases a large amount of carbon is incorporated by the process. For example, titanium carbide films have been deposited using tetraneopentyltitanium (Ti[CH.sub.2 C(CH.sub.3).sub.3 ].sub.4) at approximately 350.degree. C. The deposited Ti contained sufficient carbon to form a separate TiC phase.
Organometallic compounds have also been evaluated for other purposes. For example, in Egger, K., "Cyclopentadienyl-Metal Complexes II. Mass Spectrometric and Gas Phase Kinetic Studies on the Thermal Stability and the Structure of (CH.sub.3).sub.3 Pt-C.sub.5 H.sub.5," J. Organometallic Chemistry, 24, 501 (1970), the structure of [cyclopentadienyl(trimethyl)platinum(IV)] was reported. The article concluded that the cyclopentadienyl group was .pi. bonded. Studies of the hydrogenation of (1,5 cyclooctadiene) dimethylplatinum has also been reported. Miller, T. et al., "Heterogeneous, Platinum-Catalyzed Hydrogenation of (Diolefin)dialkylplatinum(II) Complexes: Kinetics," J. Am. Chem. Soc., 3146 (1988) (and two subsequent articles by the same author). Hydrogenation of nickelocene ((C.sub.5 H.sub.5).sub.2 Ni) has also been reported. Kaplin, Y. et al., "Decomposition of Nickelocene in Presence of Hydrogen," UDC 547.1'174, c. 1980 Plenum Publishing Corp., translated from Zhurnal Obshchei Khimii, 50, 118 (1980). None of these articles, however, report any special benefits of using these compounds in CVD.
In our newly issued patent U.S. Pat. No. 5,130,172 there is described the low temperature deposition of organometallic compounds onto substrates. The process includes coating onto various substrates such as glass or silicon, at least one metal that can readily cycle between two oxidation states and that is also a hydrogenation catalyst capable of facilitating the hydrogenation of hydrocarbon ligands of precursor organometallic compounds. During the process the substrate is maintained at a relatively low temperature in the range of about room temperature up to about 190.degree. C. depending on the substrate.
The precursor organometallic compound of our patent which is vaporized or dissolved has the general formula EQU L.sub.n MR.sub.m
wherein L is hydrogen, ethylene, allyl, methylallyl, butadienyl, pentadienyl, cyclopentadienyl, methycyclopentadienyl, cyclohexadienyl, hexadienyl, cycloheptatrienyl, or derivatives of said compounds having at least one alkyl side chain having less than five carbon atoms,
M is a metal which can readily cycle between two oxidation states and can catalyze hydrogenation of hydrocarbon ligands of the organometallic compound such as cobalt, rhodium, iridium, nickel, palladium, platinum, osmium, ruthenium.
R is methyl, ethyl, propyl, or butyl,
n is a number from 0 to the valence of said metal,
m is a number from 0 to the valence of the metal, and m plus n must equal the valence of the metal.
During the process the substrate is continuously exposed to hydrogen gas and the organometallic compound at a temperature in the range of about room temperature to about 190 .degree. C. depending on the organometallic compound. During the course of reaction, a layer of the metal from the organometallic compound is deposited on the surface of the substrate which at the same time catalytically hydrogenates the hydrocarbon ligand of the organometallic compound to provide significant purity of the deposited metal compared with prior art processes. In the above process, the best results were obtained using CpPtMe.sub.3 or MeCpPtMe.sub.3.
A particular advantage of the process is that in most cases the process takes place at a relatively low temperature and the carbon impurity can be kept as low as 3.5 mol % or less.
However, following the above process using a heated substrate of, for example, silicon or glass and an organometallic compound wherein the metal is comprised of a refractory metal such as tungsten and in the presence of hydrogen gas, it has been found that the deposited tungsten metal is contaminated significantly with carbon, as much as 30 mol % to 40 mol % depending on the reaction conditions. Thus, the presence of the hydrogen reducing gas is insufficient in the case of tungsten organometallics to prevent the contamination of the deposited tungsten film with carbon.
Thus, it is an object of the invention to provide a method for depositing metal films, and especially tungsten metal films by both liquid and gas phase CVD. Such metal films contain significantly reduced heteroatom contaminants than hitherto possible. As used herein and in the appended claims the term "liquid CVD" refers to use of a solution deposited precursor compound in the CVD process.
It is a further object of the invention to provide a thin metal film, especially tungsten having up to 5-10 mol % of another catalytic metal such as platinum, and other Group VIII transition metals such as ruthenium, osmium, cobalt, rhodium, iridium, nickel, and palladium.
It is a further object of the invention to provide a process for the deposit of tungsten and/or other metals having dissolved therein or alloyed with, or heterogeneously admixed, one or more of the above catalytic metals.
It is also an object of the invention to provide a process wherein tungsten and/or other metals can be deposited on a substrate from precursor film forming metal compounds and the purity improved by including precursor catalytic metal compounds which catalyze the removal of carbon from the deposited tungsten and other metal films and at the same time remain as a dissolved portion of the tungsten or other metal films, or at the metal surface or at grain boundaries.
It is a further object of the invention to provide a process for controlling the amount of heteroatom such as carbon in the deposit of metal films in which one precursor would introduce carbon during metal deposition while another precursor would deposit the pure metal. Such depositions can be simultaneous or sequential.
It is another object of the invention to provide metal films comprising a mixture of two metals with up to 10 mol % of a catalytic metal.
It is a further object of the invention to provide a method for the laser induced deposition of pure metal films using precursor film forming metal compounds and precursor catalytic metal compounds in the presence of hydrogen gas wherein the laser beam is directed on the substrate at an incident angle ranging from perpendicular to parallel.
It is also an object of the invention to provide a process for deposition of pure metals from precursor film forming metal compounds in the presence of precursor catalytic metal compounds and hydrogen gas wherein the precursor catalytic metal compound is first deposited on the substrate from a solvent solution followed by evaporation and thermal decomposition to leave the catalytic metal on the substrate, followed by CVD deposition of a metal from precursor film forming metal compounds in the presence of hydrogen gas. The above process is referred to herein and in the claims as "liquid CVD".
It is also an object of the invention to provide processes for the deposition of pure metal films onto substrates by various combinations of gas CVD and liquid CVD processes using precursor film forming metal compounds and precursor catalytic metal compounds in the presence of hydrogen gas.
It is also an object of the invention to selectively deposit metals onto a substrate using precursor film forming metal compounds and catalytic amounts of a precursor catalytic metal compound in the presence of hydrogen gas. In this manner, the process can be used to provide a patterned deposition of metals either from the gas phase deposition, either sequentially or simultaneously or by the liquid phase deposition of precursor compounds to deposit either the metals or catalytic metals or both or all sequentially or simultaneously.